The M2ATX will power some P4s. It all depends ont he power draw.
We compiled a FAQ that's easy to follow. I think you've been pointed at this before, but here it is again anyway: FAQ: Will <insert PSU model> Power Supply work with my system?
As far as a Dell computer, you need to make 100% certain that the Dell has a standard ATX power connector on the motherboard to use any of the ATX power supplies (Opus models, M-series and DSATX). Many dell models do not use standard ATX power connectors!!!!!!
The other thing about the P4 is that they are notorious for power draw. Some Celerons are just as bad as the P4s. The Northwood series of P4/Celeron CPUs are the biggest power hogs, and should be avoided for a vehicle installation.
The ideal choices are the Via C-series (C3 and C7) CPUs (which are typically hard-mounted to motherboards), the Mobile Turions from AMD and the newer Intel Core solo CPUs, as they are very lean on the power draw.
Will they work with your motherboard? Doubtful.
It sounds as if you have only a limited knowledge of the PC hardware that's out there, especially CPUs and motherboards. Start with the Electrical specs that are linked in that FAQ, then start doing some research about what works with what.